Understanding how proteins fold is one of the central problems in biochemistry. To investigate the mechanism of protein folding on the previously inaccessible submillisecond time scale, we have developed photochemical triggering, nanosecond laser temperature-jump, and ultrarapid mixing methods. These experiments provide the first glimpse of elementary motions in protein folding, including "-helix, $-hairpin, and loop formation, as well as collapse from the random coil state. Hairpins form in -10-6 s, -10-fold slower than helices. This result can be explained in terms of a simple model, in which hairpin formation is a continuously uphill process in free energy until favorable interactions are formed between amino acid side chains on opposite strands. Our data and analysis lead to the striking result that formation of a 16 amino acid $-hairpin exhibits most of the basic features of protein folding, including stabilization by both hydrogen bonds and hydrophobic interactions, two-state kinetic and thermodynamic behavior, and a funnel-like, partially rugged energy landscape. Comparison of the rates of helix, hairpin, and loop formation leads to an estimate of 10/6 s-1 for an upper limit on the rate of protein folding. This rate also suggests that the effective free energy barriers for the fastest folding proteins are small, and that it may be possible to observe the "downhill" folding scenario predicted by the energy landscape theory of Wolynes and coworkers. Fast protein folding studies are not only uncovering basic mechanisms, but may have additional biological significance. We have speculated that evolution selects amino acid sequences not only to form three-dimensional structures that perform a specific biological function, but also to rapidly bury hydrophobic residues in compact (but not necessarily native) structures to avoid aggregation.